1. Field of the Invention
The present invention relates to imaging sub-surface structures, particularly hydrocarbon reservoirs and fluids therein, and more particularly to electromagnetic energy sources for electromagnetic surveying of sub-surface structures.
2. Description of the Related Art
Some electromagnetic (EM) surveying systems used in geophysics provide electromagnetic energy for traveling through a subsurface hydrocarbon reservoir for electromagnetic imaging of the subsurface hydrocarbon reservoir. Multiple sources and receivers can be positioned either in a bore that extends to the subsurface hydrocarbon reservoir or an earth surface above the subsurface hydrocarbon reservoir. In this way, the direction, velocity and saturation of injected fluids (e.g. during water flood) can be monitored. The system can also be used to locate by-passed oil and detect high conductivity zones (e.g. fracture corridors and super-k zones) to provide early warning of water break-through. Such operations can assist in optimizing reservoir management, preventing oil bypass and thereby improving volumetric sweep efficiency and production rates.
Some current EM systems in geophysics include an overly large antenna in order to be able to generate a moderately low frequency signal out of a small antenna. The apparent ‘aperture’ of the antenna (wavelength to antenna size ratio) can be problematic. Some current EM systems cannot easily match impedance of the system to the geological matrix and increase transmission efficiency. Some current EM systems use high-current cable to provide power to the EM transmitter. However, these systems have been shown to have difficulty transferring a crisp high-current pulse from the power supply, down a low-loss cable, and then matching that into the antenna and in addition, the high-current cabling can also transmit a signal, which made the resulting measurements unclear.